The present invention relates to a method and an apparatus for restraining a vibration of a floor.
Floors in a building vibrate when the building vibrates due to earthquakes, winds, traffics, and other vibration sources. Floors are induced to vibrate also because of vibration sources located on the floor. Modern buildings generally have more flexibility and less damping compared to conventional buildings by virtue of affluently flexible modern structural members and precise construction techniques. Consequently, modern buildings permit larger displacements than before, and vibration in modern buildings tend to become larger than in conventional buildings. Then, excessive vibration of floors may cause such various inconveniences and problems as misoperation of equipments installed on the floor and uncomfortable feeling of motion induced to the occupants etc. Reduction of vibration is desirable also to reduce vibrational loads exerted on the structural members of the building when the vibration source is located on the floor.
In order to resolve some of these inconveniences, present inventors have already proposed a vibration restraining apparatus for buildings by a Japanese Patent Application No. 62-241045, FIG. 1. By the Patent Application, disclosed is a vibration restraining apparatus comprising a tank and a liquid retained in the tank to be installed at a roofing of the building so as to restrain vibration of the building. As the building vibrates, vibrational energy of the building excites the liquid, having a sloshing natural frequency identical to a natural frequency of the building, to slosh in resonance with the building. Consequently, vibration of the building is restrained as the vibrational energy of the building is transmitted to the liquid. It was stated in the specification that the weight of the liquid retained in the tank had better be larger than 1% of the building so as to assure an effectiveness.
A problem as to the above-mentioned conventional apparatus and methods for restraining vibration of a building is that the apparatus requires a large space to be installed in and consequently the space is found, in general, only at the top of the building.
Another problem in conventional apparatus and methods is that a large number of such tanks have to be installed. It is because that the size of the liquid tank is relatively small in order to make the sloshing frequency identical to that of the building and because effective weight of the liquid has to be larger than 1% of that of the building. Another problem is that it is not easy to make sure that a prescribed amount of liquid is retained in each of the tanks.
On the other hand, there is a requirement to suppress vibration of a specific floor where precision instruments are installed such as computers. In some cases, vibration restraint of each floor is more effective or economic, and thus preferable.
In some other cases, vibration source is located on a specific floor and the floor had better be isolated so as not to transmit vibration to the building.
A conventional technique to reduce vibration of a floor is to support the floor from structural members of the building by resilient means. By virtue of the resilience of the support, natural frequency of the floor is lowered becomes far apart from that of the building. The support of the floor acts as a kind of isolation device.
A problem in this case is that relative displacement between the floor and the building tends to become large and cause various inconveniences on pipings and cables passing through the building and the floor. Another problem is that once a vibration occurs the vibration tends to continue long before being stabilized and give uncomfortable feeling to the occupants.